1. Field of the Invention
This invention relates to conductive polymeric compositions and, more particularly, to in situ free-radical modified-carbon black filled polypropylenes having a narrowed molecular weight distribution and a lowered viscosity.
2. Description of the Related Art
Processing properties of polyolefins have been improved by techniques of narrowing their molecular weight distributions. U.K. Patent No. 1,042,178 describes processes of such narrowing wherein polyolefins are degraded at elevated temperatures by high shear gradients or by heat treatment in the presence of oxygen. U.S. Pat. No. 4,451,589 describes another such process wherein polymers are chemically degraded by free-radical scission, preferably with a peroxide, to yield a low weight average molecular weight polymer exhibiting a low viscosity. The lowered viscosity is a highly desirable property in certain polymer processes, especially the injection molding of polypropylene. Commercially available polypropylene has a molecular weight ranging upward to 1,000,000 with a molecular weight distribution of 6 to 12. Producers of polypropylene products prefer molecular weights and molecular weight distributions according to the end application. For instance, in the high speed spinning and fiber forming of polypropylene, a molecular weight of about 160,000 with a distribution of 2.5 to 4.5 is preferred, whereas in injection molding a molecular weight in the range of 150,000 to 400,000 with a molecular weight distribution of 3.5 is preferable.
U.S. Pat. No. 3,953,655 teaches a process of producing free-radical modified polyolefins by injecting peroxide into an extruder under specified conditions.
In general, the lowering of average molecular weight by peroxide treatment of polypropylene homopolymer or copolymer gives rise to decreased melt viscosity.
Techniques of causing thermoplastic materials to exhibit electrical conductivity by the addition of conductive materials such as carbon black and graphite are well known.
For instance, U.S. Pat. No. 4,717,505 teaches electrically conductive mixture of thermoplastic resins of propylene and ethylene copolymer containing conductive carbon in both carbon black and fiber form. The fibrous carbon is taught as improving the conductivity over materials containing only particulates, as well as providing a material with sufficient fluidity for injection molding of thin parts. The carbon comprises 33 to 60% of the weight of the copolymer-carbon mixture.
For certain applications, such as for use in molded articles to be surface painted rather than mixed with colorant during compounding, the incorporation of large amounts of carbon black has been found to adversely affect the paintability of the finished article. U.S. Pat. No. 4,734,450 teaches that the addition of smaller amounts of carbon, in the range of less than about 1%, to a polyolefin copolymer in combination with an inorganic filler such as talc, calcium carbonate, or barium sulfate yields a polypropylene based resin useful for making molded articles with improved surface paintability over such resins containing large amounts of carbon black alone.
Carbon black filled polypropylenes have been used for static dissipative applications which require surface resistivities in the range of 10.sup.1 -10.sup.8 ohms per square. At the typical filler loadings necessary to achieve this level of conductivity, the melt viscosity of the carbon black-polypropylene mixture is significantly higher than that of the base polypropylene resin. This causes the usual extrusion and molding problems associated with high viscosity.